Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a pre- orogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by com- parison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor dur- ing Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tecton- ic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments from the hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks direct- ly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic archi- tecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboring meta-sediments, indicating the orig- inal proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultra- mafic rocks pre-dating Alpine metamorphism, indicating that they were juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, con- tinental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumed mantle domains sampled in the rest of the Western Alps, indicates that sev- eral tectono-metamorphic units from the most deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the tran- sition between areas floored by hyper-extended crust or hydrated subcontinental mantle and domains consisting of thicker continental crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Briançonnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.